PAAD-Family Proteins and Host Defense Mechanisms. The PAAD (PYRIN) domain is a protein interaction module belonging to a superfamily of protein domains involved in NF-kappaB induction and Caspase activation, which includes the Death Domain (DD), Death Effector Domain (DED), and Caspase-Associated Recruitment Domain (CARD) proteins. At least 19 PAAD encoding genes are predicted in the human genome, and hereditary mutations in some of these genes are associated with hyper-inflammation syndromes. Fourteen PAAD-proteins (called PANs/PYPAFs) have a domain architecture reminiscent of pathogen-response genes of plants, with a PAAD, followed by a Nucleotide-binding (NACHT) domain, and then Leucine-Rich-Repeats (LRRs). Conversely, proteins consisting only of a PAAD (PAAD-Only Proteins) [POPs] are encoded within the human genome and the genomes of poxviruses, presumably operating as antagonists of PANs. Recent evidence implicates PAADs inactivation of Caspase-1, an activator of pro-inflammatory cytokines, and in regulation of NF-kappaB, a family of transcription factors that play critical roles in inflammatory and immune cell responses. Moreover, we have observed that certain PAAD-family proteins associate with the IkappaB Kinases (IKKs) that mediate NF-kappaB induction. The aim of this proposal is to provide a better understanding of the molecular mechanisms and physiological roles of PAAD-family proteins in inflammatory cell responses to infectious agents. Three PAAD-containing proteins will serve as prototypes for studies of this large protein family, including (a) ASC, a bipartite adapter protein capable of associating with both the IKK complex and pro-Caspase-1; (b) Cryopyrin, a PAN-family protein, representing the causative protein of cold autoinflammatory syndrome; and(c) the cellular and viral POPs, small proteins comprised only of a PAAD that antagonize the NF-kappaB-inducing actions of ASC and Cryopyrin. Biochemical, genetic, and cell-biology methods will be employed for gaining a better understanding of how these proteins function at the biochemical level and what functions they perform in the context of responses to infectious agents.